linux-kernel - Re: [GIT PULL] last minute percpu fix for v3.15-rc8

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Date:	Wed, 4 Jun 2014 14:12:14 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Christoph Lameter <cl@...two.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Kent Overstreet <kmo@...erainc.com>,
	Sebastian Ott <sebott@...ux.vnet.ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>
Subject: Re: [GIT PULL] last minute percpu fix for v3.15-rc8

Hello,

On Wed, Jun 04, 2014 at 12:16:56PM -0500, Christoph Lameter wrote:
> On Wed, 4 Jun 2014, Tejun Heo wrote:
> 
> > I'll ping Christoph whether __this_cpu_*() ops can be better annotated
> > so that it can trigger lockdep warning when used from multiple
> > contexts.
> 
> We just added checks that verify the proper preemption mode for
> __this_cpu operations. That did not catch it?

That one just ensures that __this_cpu_*() are called with preemption
disabled.  What's necessary is tracking whether each percpu variable
is operated on from different contexts so that things like
"inconsistent {HARDIRQ-ON-W} -> {IN-HARDIRQ-W} usage" lockdep warnings
can be triggered in cases like this.

Given that percpu operations can be performed on any allocated percpu
area, it's a bit difficult to achieve tho.  The only way I can think
of is percpu allocator creating lockdep key for all the allocated
offsets which isn't too enticing. :(

Thanks.

-- 
tejun
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